#include <bits/stdc++.h>
 
const double pi = 3.141592653589793238462643383279;
 
 
using namespace std;
//typedef
//------------------------------------------
typedef vector<int> VI;
typedef vector<VI> VVI;
typedef vector<string> VS;
typedef pair<int, int> PII;
typedef pair<long long, long long> PLL;
typedef pair<int, PII> TIII;
typedef long long LL;
typedef unsigned long long ULL;
typedef vector<LL> VLL;
typedef vector<VLL> VVLL;
 
 
//container util
//------------------------------------------
#define ALL(a)  (a).begin(),(a).end()
#define RALL(a) (a).rbegin(), (a).rend()
#define PB push_back
#define MP make_pair
#define SZ(a) int((a).size())
#define SQ(a) ((a)*(a))
#define EACH(i,c) for(typeof((c).begin()) i=(c).begin(); i!=(c).end(); ++i)
#define EXIST(s,e) ((s).find(e)!=(s).end())
#define SORT(c) sort((c).begin(),(c).end())
 
 
//repetition
//------------------------------------------
#define FOR(i,s,n) for(int i=s;i<(int)n;++i)
#define REP(i,n) FOR(i,0,n)
#define MOD 1000000007
 
 
#define rep(i, a, b) for(int i = a; i < (b); ++i)
#define trav(a, x) for(auto& a : x)
#define all(x) x.begin(), x.end()
#define sz(x) (int)(x).size()
 
typedef long long ll;
typedef pair<int, int> pii;
typedef vector<int> vi;
const double EPS = 1E-8;
 
#define chmin(x,y) x=min(x,y)
#define chmax(x,y) x=max(x,y)

//ここから編集
template<typename T = int> struct SegmentTree{

  vector<T> node;
  int N;

  SegmentTree(int n){
    N = 1;
    while(N < n) N*=2;
    node.resize(2*N-1, INT_MAX);
  }

  void update(int x, int val){
    x += N-1;
    
    node[x] = val;
    while(x > 0){
      x = (x-1)/2;
      node[x] = min(node[2*x+1], node[2*x+2]);
    }
  }

  // [a, b)について求める
  int getmin(int a, int b, int k=0, int l=0, int r=-1){

    if(r < 0) r = N;
    if(r <= a || b <= l) return INT_MAX;
    if(a <= l && r <= b) return node[k];
    
    int vl = getmin(a, b, 2*k+1, l, (l+r)/2);
    int vr = getmin(a, b, 2*k+2, (l+r)/2, r);
    return min(vl, vr);
  }


};
int main() {
  cin.tie(0);
  ios::sync_with_stdio(false);
  cout << fixed << setprecision(10);
    
  int N; cin >> N;
  SegmentTree<int> seg(N);

  vector<int> A(N);
  REP(i,N){
    cin >> A[i];
    seg.update(i, A[i]);
  }
  int ans = 1e9;
  for(int i=1; i<N-1; i++){
    int l = seg.getmin(0, i), r = seg.getmin(i+1, N);

    if(l < A[i] && r < A[i] && l != r){
      ans = min(ans, l+A[i]+r);
    }
    if(l>A[i] && r>A[i] && l != r){
      ans = min(ans, l+A[i]+r);
    }
  }
  if(ans == 1e9)ans = -1;
  cout << ans << endl;
  return 0;
}